1. Field of the Invention
This invention relates to an improved method of regulating polymer molecular weight in the preparation of homopolymers, copolymers and graft polymers of vinyl halides such as vinyl chloride, in which polymerization is carried out in bulk in the presence of a small concentration of an aldehyde devoid of ethylenic or acetylenic unsaturation without substantially diminishing the conversion rate of the polymerization.
2. Description of the Prior Art
The tendency of certain free radical polymerizable materials such as the vinyl halides to polymerize to relatively high molecular weight polymers under normal polymerization conditions is well known. These high molecular weight polymer products have relatively high fusion temperatures and/or high melt viscosities so that they are generally processed with difficulty in processing procedures which require fusion of the polymer and handling of the molten polymer, i.e. they cannot be readily processed without the use of special equipment except at temperatures so high as to have a detrimental effect on the strength and color of the processed polymer.
Various techniques have been proposed for regulating the molecular weight of such polymers, i.e. for preparing polymers of lower molecular weight and, hence, of improved processability. One such technique involves carrying out the polymerization in an inert organic diluent but such procedure is not generally practical because of the cost of the diluent and the costly inconvenience of separating the polymer product from the organic diluent. Other molecular weight regulating techniques overcome the disadvantages of solvent polymerization but are attended by disadvantages of their own. Thus raising the temperature of the polymerization, while effective in reducing polymer molecular weight involves the danger of a "runaway", i.e. excessively violent, polymerization. Alternatively it is known to regulate molecular weight by carrying out the polymerization in the presence of a reagent which acts as a chain transfer agent to limit the molecular weight of the polymer. Such chain transfer agents contain a functional group which is capable of terminating growing free radical-terminated polymer chains in the polymerization reaction mass. Usually the molecular weight regulating agent transfers an atom or group together with an unpaired electron associated therewith to the free-radical-terminated group or substituent at the end of a growing polymer chain thus stopping the growth of the chain in the polymerization reaction. The resulting free-radical species derived from the molecular weight regulator can then add to another free-radical terminated polymer chain thereby stopping the growth of the latter chain and becoming attached to the chain as an end group. Alternative to adding to polymer chain ends, the free-radical species derived from the molecular weight regulating reagent can, of course, combine with any other free-radical species in the polymerization reaction mass, for example, the free radical or radicals produced by homolytic dissociation of the reaction initiator. Many known reagents can be employed as chain transfer molecular weight lowering agents in molecular weight regulation of vinyl halide polymers, but, in general, the use of known molecular weight control agents is attended by several drawbacks and disadvantages which limit or even prohibit their practical application in polymer production. For example, many molecular weight regulating agents, such as organic mercaptans e.g. alkyl mercaptans, or organometallic compounds, e.g. tetramethyltin, are not only relatively volatile but also malodorous or toxic so that their use as molecular weight regulators in vinyl halide polymerization creates pollution and toxicity problems in locations where the polymer is made and processed.
It is known that polyvinyl halides of diminished molecular weight are obtained when vinyl halide monomer is polymerized in the presence of relatively massive concentrations of aldehydes devoid of ethylenic and acetylenic bonds as a means of preparing crystalline vinyl halide polymers. Thus J. F. Gillespie and P. H. Burleigh, Polymer Preprints 1, No. 1, p. 131-138 (1960) and the preliminary communication thereof, P. H. Burleigh, J. Am. Chem. Soc. 82 749 (1960), teach polymerization of vinyl chloride in the presence of saturated aldehydes, i.e. aldehydes devoid of ethylenic and acetylenic unsaturation, such as saturated aliphatic aldehydes and aromatic aldehydes, wherein the aldehyde and the vinyl halide monomer are charged to the polymerization in substantially equimolar proportions corresponding to a concentration of about 40 percent or more of the aldehyde based on the weight of the polymerization mass. While the aldehyde reagent lowers the molecular weight of the polymer, it also generally retards the rate of vinyl halide polymerization (see Gillespie and Burleigh, op. cit., p. 136, second paragraph). In addition to the latter serious disadvantage, polymerization of vinyl halide in the presence of the saturated aldehydes as taught by the Gillespie et al, and Burleigh references impairs the solubility of the polymer product in conventional organic solvents for polyvinyl halides. Thus conventional vinyl chloride polymer prepared in absence of an aldehyde is soluble in tetrahydrofuran and cyclohexanone at both room temperature and at moderately elevated temperatures. However the products obtained by polymerizing the vinyl chloride monomer in the presence of aldehyde according to the Gillespie - Burleigh technique are insoluble in tetrahydrofuran at concentrations in excess of 1%, even on heating, and are only soluble in cyclohexanone on heating. The aforementioned relative insolubility of the Gillespie and Burleigh polymers is particularly undesirable since such insolubility limits the utilization of the polymers in applications wherein good solubility of the polymer in organic solvents is desirable or required, e.g. solution casting processes of the type described in Modern Plastics Encyclopedia 51, No. 10A, October 1974, p. 301. The prior art technique of Gillespie and Burleigh is also highly disadvantageous in that aldehyde, employed in relatively massive amounts impedes, and in some cases, prevents recovery of the polyvinyl halide product. Thus, in the Gillespie - Burleigh procedure the amount of aldehyde employed in the polymerization is far in excess of that which can react according to the aforementioned chain transfer mechanism. Accordingly, at the end of the polymerization reaction a substantial amount of aldehyde remains which must be removed in order to recover the polymer. In other words, the Gillespie - Burleigh technique suffers from disadvantages at least as serious as those mentioned above in connection with the diluent method of molecular weight regulation. Furthermore, although aldehydes devoid of ethylenic or acetylenic multiple bonds are relatively stable, under conditions of vinyl halide polymerization, particularly at the moderately elevated temperatures conveniently employed therein, the excess aldehyde in the polymerization mass undergoes aldehyde self-condensation reactions forming low melting solids or oils, thereby exacerbating the problem of separating vinyl halide polymer from the reaction mixture. The difficulties encountered in separating the excess saturated aldehyde and/or self condensation products thereof from vinyl halide polymerization product prepared in the presence of the aforementioned massive proportions of aldehyde are illustrated by Example 6 below and by the inability of Gillespie and Burleigh to purify several polyvinyl halide products (see Gillespie and Burleigh, op. cit. Table VI).
M. Imoto et al., Makromol Chem. 48 80 (1961) also disclose that when polymerization of vinyl chloride is carried out in the presence of acetaldehyde in tetrahydrofuran as reaction solvent to impart crystallinity to the polymer, the polymer molecular weight is lowered. Although a more crystalline product is obtained as compared to that obtained by Gillespie and Burleigh, the efficiency of the Imoto et al. polymerization as measured by polymerization rate is substantially reduced by the presence of even small concentrations of the aldehyde in the reaction mixture (see FIG. 3, Imoto et al. op. cit.). Moreover the polymers produced by the solution polymerization technique of Imoto et al. have, like the Gillespie and Burleigh polymers, unsatisfactory low solubilities in conventional organic solvents such as cyclohexanone, tetrahydrofuran and dimethylformamide (see Table 1, Imoto et al. op. cit.).
U.S. Pat. No. 3,160,614 to S. P. Nemphos et al, teaches that in polymerization solvent or diluent unsaturated aldehydes in small concentration, e.g., 0.2% or more based on the weight of vinyl compound, are effective in lowering the molecular weight of the polymer. However, the aldehydes of Nemphos et al. are alpha-methyl, alpha, beta-ethylenically unsaturated aldehydes which are structural distinguished from those prescribed by Gillespie and Burleigh and Imoto et al. The Nemphos et al. unsaturated aldehydes are not only more difficult to prepare, and hence, more costly than the latter aldehydes devoid of ethylenic or acetylenic unsaturation, but also are less stable because of their olefinic unsaturation. Furthermore the aforementioned unsaturated aldehydes are capable of undergoing vinyl-type polymerization with vinyl halide monomer to produce polymers structurally distinguished from those prepared in the presence of the saturated aldehydes, i.e. the unsaturated aldehydes react at their carbon-to-carbon double bonds as comonomers with the vinyl halide so that all of the unsaturated aldehyde charged to the polymerization becomes combined within the polymer chain rather than merely at the chain ends. Since the aldehyde functional group of such unsaturated aldehyde residues can also combine to a substantial extent with polymer chain ends according to the chain transfer mechanism described above, vinyl halide polymers prepared in the presence of the unsaturated aldehydes are further distinguished structurally from the corresponding polymers prepared in the presence of the saturated aldehydes in being crosslinked. Moreover, ethylenically unsaturated aldehydes, when charged to vinyl halide polymerizations even in small amounts, e.g. 0.1% based on the weight of vinyl halide, have so great a retarding effect on the polymerization as to terminate the reaction as disclosed in U.S. Pat. No. 2,616,887 to M. H. Danzig, et al.
W. L. Semon, U.S. Pat. No. 1,983,949 teaches polymerization of vinyl chloride in bulk in liquid phase at 100.degree. C. in the presence of a free radical initiator, such as benzoyl peroxide, and an aldehyde reagent which is either the ethylenically unsaturated aldehyde, acrolein, of the type mentioned above or the one carbon atom-containing aldehyde, formaldehyde. The latter aldehyde is distinguished from all higher aldehydes in being devoid of a carbon to carbon bond and in its almost explosive proclivity to polymerize to a stable polymer at 100.degree. C in bulk liquid phase as disclosed by J. Furukawa and T. Saegusa, "Polymerization of Aldehydes and Oxides", Interscience Publishers, 1963, p. 58, lines 12-24, p. 77, lines 24-26 and p. 112, line 21 and last two lines. The result of the Semon process, however, is a vinyl chloride polymer of higher than normal molecular weight and insolubility in all common solvents, i.e., a result contrary to the teachings of this invenion.